$ tmux -V
tmux 2.1

$ tmuxp -v
tmuxp 0.9.1

$ tmux list-sessions
2: 1 windows (created Tue Oct 20 15:17:58 2015) [140x45] (attached)

$ tmux has-session -t pomp
can't find session pomp

$ cat pomp.yml | grep session_name
session_name: pomp

$ tmuxp load pomp.yml -2
load /home/user/prj/pomp.yml
(I) [15:10:01] tmuxp.cli cli.load_workspace():270 Loading /home/user/prj/pomp.yml.
Session name pomp is already running. Attach? [Y/n] Y
Traceback (most recent call last):
  File "/usr/lib/python3.5/site-packages/tmuxp/cli.py", line 271, in load_workspace
    builder.build()
  File "/usr/lib/python3.5/site-packages/tmuxp/workspacebuilder.py", line 119, in build
    self.sconf['session_name']
tmuxp.exc.TmuxSessionExists: Session name pomp is already running.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/tmuxp", line 9, in <module>
    load_entry_point('tmuxp==0.9.1', 'console_scripts', 'tmuxp')()
  File "/usr/lib/python3.5/site-packages/tmuxp/cli.py", line 968, in main
    command_load(args)
  File "/usr/lib/python3.5/site-packages/tmuxp/cli.py", line 420, in command_load
    load_workspace(file_cwd, args)
  File "/usr/lib/python3.5/site-packages/tmuxp/cli.py", line 292, in load_workspace
    builder.session.switch_client()
AttributeError: 'NoneType' object has no attribute 'switch_client'